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  • Dec 2017. Our VECSEL makes the cover of IEEE Journal of Selected Topics in Quantum Electronics.
  • Sept. 2017. Our paper ”High performance terahertz metasurface quantum-cascade VECSEL with an intra-cryostat cavity” is chosen as “Editor’s Pick” in Applied Physics Letters. Link.
  • June 2017. Congratulation to Luyao Xu for receiving her Ph.D., Jiawei Wang and Yujie Lin for receiving their M.S. and to Daguan Chen for receiving his B.S.!
  • May 2017. Luyao Xu has been awarded the UCLA EE Department “Distinguished PhD Dissertation Award in Physical & Wave Electronics” for 2016-17. Congratulations!
  • April 2017. We report metasurface lasers with electrically switchable polarization in Optica. Press release.
  • April 2017. Congratulations to incoming graduate student Parastou Mortazavian on receiving the NSF Graduate Research Fellowship! Link.
  • October 2016. We report high brightness and efficiency THz QC VECSELs with focusing and amplifying metasurfaces. Link.
  • October 2016. Our group is featured in UCLA Engineer Magazine. Link
  • Prof. Williams will give an invited talk at Photonics West 2017, and invited tutorial at OTST 2017. A video of the Photonics West talk is available here.
  • Sept 2016. Congratulations to Dr. Benjamin Burnett for successful completion of his Ph.D. !
  • June 2016. Congratulations to Luyao Xu for being awarded the UCLA Dissertation Year Fellowship for 16/17!
  • Feb 2016. Prof. Williams is awarded a Presidential Early Career Award for Scientists and Engineers (PECASE). Link Link
  • Feb 2016.Congratulations to Luyao Xu for winning the Best Student Presentation Award in the VECSEL Conference at SPIE Photonics West 2016! Link
  • Jan 2016. Press coverage on metasurface VECSEL in Physics Today, Laser Focus World, Photonics.com, Semiconductor Today,
  • September 2015. We report at ITQW 2015 the first demonstration of an external cavity laser based upon an amplifying reflectarray metasurface - a THz VECSEL. It is published in  Applied Physics Letters.
  • 2015. Congratulations to Luyao Xu for winning a 2015 SPIE Optics and Photonics Scholarship!
  • May 2013. Congratulations to Amir Ali Tavallaee, for winning the EE department’s “Distinguished PhD Dissertation Award in Physical & Wave Electronics” for 2012-13!
  • March 2013. Our paper on “Radiation Model for THz  Transmission-Line Metamaterial QC-Lasers” wins 2013 IEEE Trans. THz Science and Technology Best Paper Award.
  • January 2013. We report the first THz active composite right/left handed (CRLH) metamaterial waveguides in Applied Physics Letters.
  • January 2012. Prof. Williams wins NSF CAREER award.
  • April 2011. Our upcoming talk on active THz metamaterial waveguides at SPIE Defense, Security, and Sensing (DSS) Conference is featured in the SPIE Newsroom (April 11 2011).
  • July 2010. Our proposal for THz metamaterial lasers has been covered in Nature Photonics "Research Highlights”.
  • March 2008. Prof. Williams wins DARPA Young Faculty Award.
  • September 2007. Nature Photonics publishes a review paper on THz quantum-cascade lasers.

Research Interests and Projects

THz Quantum-Cascade Lasers

    Quantum cascade (QC) lasers are based upon engineering semiconductor quantum wells with the precision of a single atomic monolayer to provide materials which exhibit optoelectronic properties not found in nature. Specifically, the engineering of "artificial molecules” to control wavefunctions, electronic energies, electron tunneling, and electron scattering probabilities allows us to develop semiconductor lasers in the THz frequency range - one of the last unconquered spectral regions. THz QC-lasers are the only solid-state source of continuous-wave radiation in the 1-5 THz range that can provide milliwatts or more of output power. Ongoing research involves engineering of novel designs for high-temperature, high-power, and low-frequency operation. We collaborate with several groups for the growth of laser material  via molecular beam epitaxy.

Resonant-phonon QC-laser active region

Image of metal-metal THz QC-laser waveguide

A review paper on THz quantum-cascade lasers.

Other selected papers

Metasurface reflectarray external cavity lasers

    We (and our collaborators) are investigating active reflectarray “metasurfaces” composed of sub-wavelength antenna elements embedded with quantum-cascade laser gain material for a variety of laser configurations. We have demonstrated a “Vertical External Cavity Surface Emitting Laser (VECSEL)” in the terahertz range for the first time, by using a reflecting metasurface made up of sparse metal-metal waveguide antenna coupled microcavities loaded with QC-laser material. This allows high-power, high efficiency surface emission with a high-quality beam. 

Selected publications:

Metasurface lasers with polarization control

    We have developed a new type of metasurface laser where the output polarization can be switched via electrical control. This is based upon a “polarimetric” amplifying metasurface made up of two sets of interleaved antennas loaded with QC-laser gain material. Each set amplifies a different polarization state, which in turn controls the polarization of the laser cavity mode depending upon which set receives current injection.

Selected publications:

Active Terahertz Metamaterial Waveguides and Antennas

    Electromagnetic metamaterials are constructed from dielectric, metallic, and plasmonic elements with deep subwavelength dimensions engineered to provide properties not found in nature - such as negative refractive index, or optical magnetism. We are investigating the development of metamaterials that are integrated with THz gain media and other active structures that will enable novel optoelectronic devices and functionality, such as laser cavities that exhibit zero index of refraction, highly directional THz antennas, materials and devices with dynamically tunable characteristics. This work is a collaboration with Prof. Tatsuo Itoh and Northrop Grumman Aerospace Systems.

Simulated electric field inside a metamaterial zero index laser cavity

THz passive composite right/left handed metamaterial waveguide metasurface


Simulation of CRLH metamaterial antenna excited at the zero-index frequency

THz active  composite right/left handed metamaterial waveguide leaky wave antenna

Selected publications:

Intersubband/level transport and optical properties in low-dimensional semiconductors

    The electronic transport and optical properties of quantum-cascade lasers and future quantum-dot intersublevel devices involve a complex interplay of coherent and incoherent transport and scattering mechanisms, including resonant tunneling, electron-phonon scattering, electron- impurity and electron-interface roughness scattering, in addition to coherent and nonlinear interactions with the electromagnetic field. We have developed formalism and simulation tools based upon a density matrix approach for handling a large number of states/subbands

Selected publications:

Micromachined and integrated antennas for THz QC-lasers

    The best performing THz QC-lasers use metal-metal waveguides with dimensions (~10 mm) much smaller than the wavelength (l~60-300 mm). While these waveguides provide excellent temperature and threshold performance, their sub-wavelength dimension results in highly divergent beams and highly reflective facets, which lead to sub-optimal coupling efficiencies. In other words, only a small fraction of the generated THz photons are usable. We are investigating micromachined waveguides and integrated antennas suitable for use with THz QC-lasers to obtain improved beam quality, coupling efficiency, and power output.

    Terahertz quantum cascade laser with integrated transmission-line metamaterial leaky-wave antenna

    Cavity model for TM01 mode leaky wave radiation (left) and predicted fan beam pattern (right)

Selected publications:

Semiconductor Nanowire Heterostructures for THz and infrared optoelectronics

    Semiconductor nanowires with axial and core-shell heterostructures are excellent candidates for future optoelectronic materials in the THz and mid-IR spectral ranges. We are interested in optoelectronic devices based upon intersubband/level transitions that take place between states formed by the growth of quantum wells and quantum dots within nanowires. Such structures offer unique opportunities for quantum engineering of electronic and phonon states that may lead to high efficiency and high temperature lasers, emitters, detectors in the infrared and THz spectral ranges. This work is a collaboration with the group of Prof. Diana Huffaker at UCLA

    Plasmonically enhanced nanowire array photodetector

Selected publications:

High-Efficiency Mid-infrared Quantum-Cascade Lasers

    Our group is investigating high efficiency and high power quantum-cascade lasers that operate in the mid-IR spectral range (3.8-12 mm).

Selected publications:


UCLA Henry Samueli School of Engineering and Applied Science Fellowship

National Aeronautics and Space Administration (NASA)

Defense Advanced Research Projects Agency (DARPA)

National Science Foundation (NSF)

Jet Propulsion Laboratory (SURP Program)

Center on Functional Engineered Nano Architectonics (FENA)

Army Research Office (ARO) STTR program

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